Difference between revisions of "VNC"

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== Launch VNC client ==
 
== Launch VNC client ==
  
After this you can start your favorite VNC client (viewer) to see what's inside the server. The latter would be visible on port 5925 (:25 argument). Launch a terminal, then some application. It will attach to X and you'd see it on the screen.
+
After this you can start your favorite VNC client (viewer) to see what's inside the server. The latter would be visible on port 5925 (:25 argument).
 +
[[File:Vnc.jpg]]
 +
Launch a terminal, then some application. It will attach to X and you'd see it on the screen.

Revision as of 09:28, 4 April 2013

Here's the step-by-step description of how to C/R a VNC server with some app inside.

Start VNC server and application

This should be done carefully.

  • First, X software uses SystemV IPC shared memory to exchange data between server and application, thus you'll have to run the whole stuff in IPC namespace;
  • Second, in order to be restored reliably, it's recommended to run VNC server, window manager and application in a PID namespace;
  • Third, when started from shell they will inherit session ID and terminal from that shell, which might block dump.

That said, we recommend you to run VNC and others using the program below:

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <sys/mount.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <sys/param.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <signal.h>
#include <sched.h>

#define STACK_SIZE	(8 * 4096)

static int ac;
static char **av;
static int ns_exec(void *_arg)
{
	int fd;

	fd = open("newns.log", O_CREAT | O_TRUNC | O_RDWR | O_APPEND, 0600);
	if (fd >= 0) {
		close(0);
		dup2(fd, 1);
		dup2(fd, 2);
		close(fd);
	}

	setsid();
	execvp(av[1], av + 1);
	return 1;
}

int main(int argc, char **argv)
{
	void *stack;
	int ret;
	pid_t pid;

	ac = argc;
	av = argv;

	stack = mmap(NULL, STACK_SIZE, PROT_WRITE | PROT_READ,
			MAP_PRIVATE | MAP_GROWSDOWN | MAP_ANONYMOUS, -1, 0);
	if (stack == MAP_FAILED) {
		fprintf(stderr, "Can't map stack %m\n");
		exit(1);
	}
	pid = clone(ns_exec, stack + STACK_SIZE,
			CLONE_NEWPID | CLONE_NEWIPC | SIGCHLD, NULL);
	if (pid < 0) {
		fprintf(stderr, "clone() failed: %m\n");
		exit(1);
	}
	return 0;
}

it will create necessary namespaces, will create a new session and will redirect stdio to newns.log file. To run the vnc server and an application in it you can use this script:

#!/bin/bash
set -m
Xvnc :25 -v -geometry 800x600 -i 0.0.0.0 -SecurityTypes none &
pid=$!
trap "kill $pid; wait" EXIT
sleep 3
DISPLAY=:25 $@

with the above "software" the server can be launched like this:

# ./newns ./vnc_server.sh icewm

After this you will see the process tree like below:

17854 ?        Ss     0:00 /bin/bash ./vnc-server.sh icewm
17855 ?        Sl     0:00  \_ Xvnc :25 -v -geometry 800x600 -i 0.0.0.0 -SecurityTypes none
17863 ?        R      0:00  \_ icewm

Launch VNC client

After this you can start your favorite VNC client (viewer) to see what's inside the server. The latter would be visible on port 5925 (:25 argument). Vnc.jpg Launch a terminal, then some application. It will attach to X and you'd see it on the screen.